Magnetic field is thought to play a crucial role in the formation and evolution of the large-scale structure of the Universe. Long-wavelength radio polarimetry is one of only a few possible methods to explore magnetic field in the cosmic web. Fast radio burst (FRB) is a new radio transient of millisecond duration. The origin of FRB is under discussion, although it is likely extragalactic because the dispersion measure is much larger than the Milky Way value. The fact that some sources exhibit linearly-polarized emission motivates us to utilize them for exploring the magnetized cosmic web, where Faraday tomography, the new standard technique of modern broadband radio polarimetry, is a key to distinguish the cosmic web from foreground. Faraday tomography is promising in the era of Square Kilometre Array (SKA). In this talk, I will briefly review the above science.